A stent is an implantable medical device comprising a tubular scaffold used to mechanically retain the patency of a lumen in which it is placed. Stents may be modified to provide therapeutic drugs at the site of implantation. That is, a stent can be coated with a polymer impregnated with a drug and, when the stent is in place, the drug elutes from the polymer. A variety of polymers can be used to coat stents. Poly(ethylene-co-vinyl alcohol) (EVAL), is a commercial polymer that has been found to be useful for this purpose.
EVAL possesses desirable coating characteristics such as relative impermeability to oxygen, biocompatibility, and adherence to variety of substrates such as, without limitation, stainless steel. It also, however, has drawbacks such as limited solubility in organic solvents and too great an affinity for water. Its limited solubility requires use of strong polar solvents, such as dimethylacetamide (DMAC) or dimethylsulfoxide (DMSO), to dissolve it. These solvents have high boiling points and are difficult to remove during the drying step of the coating process. EVAL's affinity for water can also be problematic in connection with its use as an effective drug carrier. For example, a commonly used grade of EVAL having about 44 mole percent ethylene groups can absorb 5 mass percent of water. This results in swelling of the polymer, which increases its porosity and, as a result, alters the rate of diffusivity of drugs out of the polymer.
What is needed is a polymer that retains EVAL's favorable characteristics but is more soluble in organic solvents so as to be more amenable to manufacturing and has less of an affinity for water so as to be more useful in the controlled release of drugs in vivo. The present invention provides such polymers.